The effect of calcium ion, pH, oxygen binding and hydrostatic pressure on the reversible association-dissociation equilibria of lobster hemocyanin are being investigated. The primary tools for this study are the ultracentrifuge, a pressure-loading chamber of the type designed by Schumaker and colleagues, and a Cary-14 spectrophotometer. Since all of these processes appear to be coupled, it will be necessary to buffer all of the variables except one, and to study the specific effect of each variable, one at a time. It is hoped in this way to learn more about the respiratory function of the hemocyanin molecule under conditions approaching those corresponding to the animal in its natural environment, in which at least hydrostatic pressure probably plays a natural role as a regulator of oxygen transport.